This invention relates to a paper secured against forgery and to a device for checking the authenticity of such papers. By the term "paper", there are to be understood all papers or other information carriers representing an ideal value independently of their material value, and whose imitation, by unauthorized persons, must be rendered as difficult as possible. In this sense, "papers" as used herein, are bank notes, shares, checks, deeds, identity cards, credit cards, contracts, and even works of art.
In order to make papers safe against forgery, it is known to provide, in the paper, safety features, such as safety threads, water marks and the like, with care being taken that any imitation of the safety features is possible only at a high cost, that is, at a cost which is higher than the ideal value represented by the paper.
However, as the many forgeries, particularly of bank notes, show, it has clearly been impossible so far to afford satisfactory safety against forgery. An investigation of the known methods and measures for securing paper against forgery has shown that, in general, these known methods and measures are relatively easy to detect, by the forger, on the paper itself, and thus are relatively easy to imitate.